Mercury in southern legal Amazonia: evaluating Prochilodus nigricans (Agassiz, 1829) as a bioindicator species and the risk of its consumption

Abstract Mercury (Hg), a highly toxic metal, continues to be widely used in artisanal and small-scale gold mining (ASGM), particularly in the Amazon region. Its improper use and disposal contribute to the contamination of aquatic ecosystems, impacting fish and the human populations that rely on them for food. This study evaluated mercury contamination, measuring total mercury (THg) in the Peixoto de Azevedo River Basin, located in the southern Brazilian Legal Amazon, by analyzing scale, skin, muscle, and liver tissues of Prochilodus nigricans , a key species in the local diet. Mercury was detected in all tissues and across all river stretches, with higher levels observed near mining sites, suggesting the association between ASGM and environmental contamination. The liver showed the highest THg levels (1.581 mg kg⁻ 1 ), followed by scales (0.537 mg kg⁻ 1 ), skin (0.153 mg kg⁻ 1 ), and muscle (0.087 mg kg⁻ 1 ). The results obtained for scales indicate their potential as a bioindicator matrix, allowing the use of less invasive sampling approaches for the species. Although muscle THg levels were below the FAO legal threshold (0.5 mg kg⁻ 1 ), the estimated health risk (THQ > 1) suggests potential harm to riverine populations due to high fish consumption. These findings underscore the urgent need for continuous monitoring and effective mitigation strategies to protect both aquatic ecosystems and public health in ASGM-affected regions.